1. Field of the Invention
The present invention relates to an inorganic scintillator.
2. Related Background of the Invention
In an apparatus used for Positron Emission (computed) Tomography (hereinafter, “PET”), the optical characteristics (wavelength conversion characteristics, etc.) of the scintillator mounted therein has a major effect on the imaging performance of the apparatus, and therefore improvement in the optical characteristics of the scintillator is the most important factor for enhancing the imaging performance of such apparatuses. Researchers are therefore actively exploring scintillator materials which can be used to construct scintillators with excellent optical characteristics, and are developing manufacturing techniques such as crystal growth techniques for realizing such scintillators.
In the field of high-energy physics as well, experiments for detection and analysis of high-energy microparticles arriving to earth from outer space require implementation of scintillators which allow efficient detection of high-energy microparticles.
Scintillators mounted in PET apparatuses include those which require high fluorescent output, those which require short fluorescent decay times and those which require high energy resolution. Particularly from the standpoint of relieving the burden on subjects being examined by PET, the examination time per subject must be shortened and therefore scintillators with short fluorescent decay times are desired.
The time-dependent change in outputted fluorescent pulse intensity for a radiation pulse entering a scintillator will now be explained. FIG. 1 is a graph schematically showing a typical time-dependent change in fluorescent pulse intensity. The fluorescent pulse intensity rises relatively steeply up to the maximum value Imax, and decays thereafter. Throughout the present specification, the term “fluorescent lifetime” will be used to refer to the time from point (0) at which the intensity of outputted fluorescence is at 10% of the maximum value (Imax) (0.1 Imax), to the point at which fluorescence is no longer observed. The phrase “time integrated value of the fluorescent pulse intensity” will mean the time integrated value of the fluorescent pulse intensity from the point at which the fluorescent intensity is at Imax (tmax) to the point at which fluorescence is no longer observed (the shaded section in FIG. 1).
As scintillators designed for shorter fluorescent decay times there are known, for example, inorganic scintillators having a construction comprising Ce (cerium) as a luminescent center in a matrix material composed of a compound metal oxide containing a lanthanoid (for example, see Japanese Examined Patent Publication No. 62-8472). Examples of known inorganic scintillators include scintillators having a chemical composition represented by the general formula: CeαLn2-αSiO5 or the general formula: CeβLn2-βAlO3, where 0<α<0.1, 0<β<0.1, and Ln (lanthanoid) represents Sc (scandium), Y (yttrium), La (lanthanum), Gd (gadolinium) or Lu (lutetium).
In particular, scintillators having a chemical composition represented by the general formula: CeαLn2-αSiO5 have high fluorescent output and are therefore widely employed for PET. As specific examples of such inorganic scintillators there may be mentioned “ALLEGRO™” by Philips Medical Systems which employs CeαGd2-αSiO5 and “ECAT ACCEL™” by Siemens which employs CeαLu2-αSiO5.
Japanese Examined Patent Publication No. 7-78215 discloses a single-crystal scintillator represented by the general formula: Ceα(LuγGd2-γ)2-αSiO5. Also, in Japanese Patent Application Laid-Open No. 2001-524163 (also in Journal of Crystal Growth 174(1997), p. 331-336) it is attempted to reduce the Lu content ratio by including Ta (tantalum), W (tungsten), Ca (calcium) and F (fluorine) in a single-crystal scintillator represented by Ceα(LuγGd2-γ)2-αSiO5.